More than 80 uncounted ballot papers for the seat of Narungga have been uncovered, the Electoral Commission of South Australia has confirmed.
One Nation's Chantelle Thomas has already been declared the winner of the seat, claiming it by a margin of just 58 votes.
Every candidate has been notified a further count will now be held on Friday "to ensure the integrity of the Narungga result", the SA electoral commission said.
"The result has already been declared," Deputy Electoral Commissioner Leah McLay told reporters on Thursday.
"The purpose of the count is for the commission to determine whether the result would have differed had those ballots been included in the initial count and the subsequent recount."
The commission is expected to seek legal advice and petition the Court of Disputed Returns if the new count changes the election result.
The uncounted ballots were in three sealed boxes and returned to the electoral commission, Ms McLay said.
"We have not investigated what the cause of the error was," she said.
Ms McLay said the SA electoral commission would support any independent external review into the 2026 state election.
Ms Thomas was declared the winner of Narungga earlier in April over the Liberal Party's Tania Stock following a recount due to the tight voting margin.
It gave One Nation four seats following the March 21 state election in a stunning result for the far-right party.
25°C